A drawback of the techniques described above is the fact that the reciprocating movement of the energy source may lead to defects being introduced into the deposited layers at the reversal points. Such defects are called “taper”, in which connection a distinction is furthermore made between geometric taper and optical taper. The term geometric taper is to be understood to mean that the thickness of the total deposition, viz. all glass layers, is not constant over the length of the tube. The term optical taper is to be understood to mean that the optical properties, which properties are mainly determined on the fibre that is eventually drawn, are not constant over the length of the preform. Optical taper is to a certain extent caused by variations in the layer thickness, but it is mainly determined by variations in the refractive index, or refractive index profiles, along the length of the preform. To achieve an adequate control of the optical properties of the fibres to be produced, an adequate control of the geometric taper is required, but in addition to that the variations in the refractive index contrast Δ should be as small as possible over the greatest possible length of the preform.
Taper has this drawback that the useful length of the preform is limited, which means that a smaller amount of fibre can be obtained from one preform. In addition to that, the properties of an optical fibre may not be constant over the length of the fibre as a consequence of said taper. It may furthermore be mentioned in this connection that if the optical properties of a fibre of great length are not sufficiently constant due to the presence of taper, the manufacturer will have to give certain guarantees with regard to the issued product certificate; in principle each individual part of the optical fibre must be in conformity with the provided specification at all times, in particular if the optical properties are re-measured, for example by the user.
U.S. Pat. No. 4,741,747 relates to a method of manufacturing optical fibres, wherein the so-called end taper is reduced by moving the plasma nonlinearly as a function of time in the region of the reversal point and/or by varying the intensity of the plasma along the length the glass tube.
U.S. Pat. No. 4,857,091 relates to a method of manufacturing optical fibres, in which a number of parameters are mentioned that influence the axial position of the local deposition zone with respect to the plasma generator, which parameters include:
(i) periodically varying the microwave power,
(ii) periodically varying the pressure in the substrate tube, and
(iii) periodically varying the stroke velocity of the resonator being reciprocated over the tube. Further details regarding i)-iii) are altogether lacking.
European patent application No. 0 038 982 relates to a method of manufacturing optical fibres wherein the plasma generator is moved along the substrate tube, which plasma generator produces a hot zone such that the hot zone can be considered as a so-called “tandem hot zone” which includes at least two zones, viz. zone I and zone II. Although it is mentioned in said document that deposition rates or deposition compositions can be changed so as to prevent the occurrence of so-called taper ends, it is not indicated in said document what specific operations such a treatment involves.
European patent application No. 0 333 580 relates to a device for manufacturing preforms for optical fibres in which a variable power microwave generator is used, in which no use is made of a non-isothermal plasma which is reciprocated between two reversal points along the substrate tube, however.
International application WO 03/054245, which was filed earlier but published later, relates to a special PCVD technique for producing preforms; no information is provided in said document with regard to varying the velocity at which the plasma is moved along the hollow substrate tube.
From British patent publication GB 2 118 165 there is known a method of manufacturing an optical fibre in which the velocity of the heat source being moved axially along the substrate tube is in accordance with a specific mathematical equation, wherein the velocity of said heat source along the tube is a function of the position of said heat source along the substrate tube, so that the total deposition thickness of the glass layers is substantially constant over the length of the tube.
From U.S. Pat. No. 5,188,648 to the present applicants there is known a method of manufacturing optical fibres wherein the movement of the plasma is interrupted each time the plasma reaches the reversal point near the gas entry point of the substrate tube, whilst the glass deposition continues, wherein the period during which the plasma movement is interrupted is at least 0.1 second.